1. Field of Invention
The field of the currently claimed embodiments of this invention relates to triboelectric x-ray sources.
2. Discussion of Related Art
Triboelectricity has been utilized in fundamental scientific research as a source of high electrostatic potential for over three centuries from the early electrostatic apparatus of Haukesbee (F. Haukesbee, Physico-Mechanical experiments on various subjects (London: 1709)) through to the eponymous generators of van der Graaf, yet there remains a notable absence of a first principles approach to the subject (M. Stoneham, Modelling Simul. Mater. Sci. Eng. 17, 084009 (2009)). Electrostatic generators store the integrated charge that is developed when two materials are rubbed together in frictional contact. The materials are selected to be furthest apart in the triboelectric series—an empirically derived list showing both the propensity of the materials to charge and the polarity of charge (P. E. Shaw, Proc. R. Soc. Lond. A 94, 16 (1917)). At the point of contact between the two materials, the frictional electrification may be of such magnitude that it may ionize the gas surrounding it, creating triboluminescence. The triboluminescence observed during peeling pressure sensitive adhesive (PSA) tape has long attracted scientific attention (E. N. Harvey, Science 89, 460 (1939)) and has an electrostatic origin. When the tape is peeled, charge densities 1012 e cm−2 (where is e is the fundamental charge on the electron) are exposed on the surfaces of the freshly peeled region and subsequently discharge (C. G. Camara, J. V. Escobar, J. R. Hird and S. P. Putterman, Nature 455, 1089 (2008)). If the tape is peeled in vacuum ˜10 mTorr, it has been found that the triboluminescence produced extends to X-ray energies (V. V. Karasev, N. A. Krotova and B. W. Deryagin, Dokl. Akad. Nauk. SSR 88 777 (1953)). More recently (Camara, et al., id.), it was found that there are two timescales for tribocharging during the peeling of tape in vacuo: the first, common to electrostatic generators and classic electrostatic experiments (W. R. Harper, Contact and frictional electrification, (Oxford University Press, London, 1967)), is the long timescale process which results in an average charge density of 1010 e cm−2 being maintained on the surface of the tape and second, a nanosecond process with charge densities of 1012 e cm−2. In addition, it was found that the X-ray discharge from peeling tape was sufficiently self-collimated at the peel line to resolve the inter-phlangeal spacing of a human digit. The emission of nanosecond X-ray pulses allowed an estimate of the emission region to be calculated. Subsequent research on peeling PSA tape with a width of 1.5 mm has confirmed that the process takes place at dimensions less than 300 μm (C. G. Camara, J. V. Escobar, J. R. Hird and S. P. Putterman, Appl. Phys. B 99, 613 (2010)). This result has provided the prospect of multiple-element X-ray sources consisting of sub-mm arrays powered by the triboelectric effect.
Underpinning this recent work on triboelectricity is a resurgence of interest in how charge transfer occurs between different materials and particularly between polymers. Particularly intriguing is the report of like-polymers charging each other (M. M. Apodaca, P. J. Wesson, K. J. M. Bishop, M. A. Ratner and B. A. Grzybowski, Angew. Chem. Int. Ed. 49, 946 (2010)). More fundamentally, an open question is whether the transfer particle is an ion (L. McCathy and G. M. Whitesides, Angew. Chem. Int. Ed. 47, 2188 (2008)) or an electron (Harper, id.)—a matter that is still debated despite centuries of experimental research. Whether the charge carriers responsible for tribocharging are electrons or ions, what is clear is that very large charge densities are readily generated.
For the most effective charging to occur, intimate contact between the materials and cleanliness of the contacting surfaces is important (R. Budakian, K. Weninger, R. A. Hiller and S. P. Putterman, Nature 391, 266 (1998)). While the peeling geometry of PSA tapes is mathematically elegant (A. D. McEwan and G. I. Taylor, J. Fluid Mech. 26, 1 (1966)) and meets both criteria, a disadvantage of using these for a portable X-ray device not requiring a high voltage supply is, however, the significant out-gassing that occurs during peeling off-the-shelf tape in vacuo (E. Constable, J. Horvat and R. A. Lewis, Appl. Phys. Lett. 97, 131502 (2010)). There thus remains a need for improved triboelectric x-ray sources.